The invention relates to an electrooptical system
which between 2 electrode layers contains a liquid crystal mixture and a further optically transparent polymer medium, PA1 whose liquid crystal molecules have an irregular orientation in the switched-off state, PA1 in which one of the refractive indices of the liquid crystal mixture is substantially the same as the refractive index of the medium n.sub.m and/or in which the quotient from the weight of the liquid crystal mixture and the weight of the optically transparent medium is 1.5 or more, PA1 which in one of the switching states has reduced transmission compared with the other state essentially independent of the polarization of the incident light, and PA1 whose liquid crystal mixture contains at least one dielectrically neutral and at least one dielectrically positive liquid crystalline or monotropic compound. PA1 GH-indicating systems, the spectrum extending from simple segment displays to displays, to which it is possible to apply any desired electrode pattern using conventional printing techniques. Applications: motor vehicle, large displays, advertizing boards, clocks PA1 displays having a high information content controlled by active or passive matrix PA1 projection systems PA1 switches PA1 which between 2 electrode layers contains a liquid crystal mixture and a further optically transparent polymer medium, PA1 whose liquid crystal molecules have irregular orientation in the switched-off state, PA1 in which one of the refractive indices of the liquid crystal mixture is substantially the same as the refractive index of the medium n.sub.M and/or in which the quotient from the weight of the liquid crystal mixture and the weight of the optically transparent medium is 1.5 or more, PA1 which in one of the switching states has reduced transmission compared with the other state essentially independent of the polarization of the incident light, PA1 whose liquid crystal mixture contains at least one dielectrically neutral and at least one dielectrically positive liquid crystalline, enantiotropic or monotropic compound, characterized in that the liquid crystalline mixture contains less than 15% by weight of one or more dielectrically positive liquid crystalline, enantiotropic or monotropic, two-ring compounds in order to lower the solubility of the liquid crystal mixture in the medium.
Depending on the mass content of the liquid crystal mixture in the system, the liquid crystal mixture can be embedded in the optically transparent medium in liquid crystal microdroplets which are separated to a greater or lesser extent from one another or else form a more or less coherent, continuous phase in which the optically transparent medium is present, for example, in the form of particles. A continuous phase is also obtained, for example, if the optically transparent medium forms a sponge-like, 3-dimensional network whose pores, in which liquid crystal is located, merge into each other to a greater or lesser extent. The expression liquid crystal microdroplets here indicates small liquid crystal compartments separated from one another which, however, in no way have to have a spherical shape, but can be irregular shaped and/or deformed.
If the optically transparent medium contains liquid crystal microdroplets, wit is described in the following as a matrix; on the other hand, if a more or less continuous phase of the liquid crystal is present, the medium is described by the expression network.
NCAP and PDLC films (NCAP=nematic curvilinear aligned phases, PDLC=polymer dispersed liquid crystal) are examples of electrooptical liquid crystal systems in which the liquid crystal is embedded in the matrix in the form of microdrops. NCAP films are usually obtained by intimately mixing the encapsulated polymeric material, such as, for example, polyvinyl alcohol, the liquid crystal mixture and a carrier material, such as, for example, water, in a colloid mill. The carrier material is then removed, for example by drying. An appropriate process is described in U.S. Pat. No. 4,435,047. In contrast, the liquid crystal mixture is first homogeneously mixed with monomers or oligomers of the matrix-forming material in the preparation of PDLC films described, for example, in U.S. Pat. No. 4,688,900, Mol. Cryst. Liq. Cryst. Nonlin. Optic, 157, (1988), 427-441, WO 89/06264 and EP 0,272,585. The mixture is then polymerized and the phase separation is induced (so-called PIPS technology; polymerization-induced phase separation). In addition, differentiation must further be made between TIPS (temperature-induced phase separation) and SIPS (solvent-induced phase separation) (Mol. Cryst. Lyq. Cryst. Inc. Nonlin. Opt. 157 (1988) 427) .
The PN system (PN=Polymer Network) described in EP 0,313,053 has a sponge-like network structure of the optically transparent medium. The content of the liquid crystal mixture-in the material of the light-modulating layer is in general greater than 60% in systems of this type and is, in particular, between 70 and 90%. In order to prepare the PN systems, a mixture of the liquid crystal, monomers or oligomers of the material forming the 3-dimensional network and a polymerization initiator, in particular a photoinitiator, is customarily brought between 2 substrate plates provided with electrodes and then polymerized, for example by light irradiation.
The liquid crystal in general has a positive dielectric anisotropy .DELTA..epsilon. and a relatively high optical anisotropy. In microdroplets matrix systems, one of the refractive indices of the liquid crystal, customarily the ordinary refractive index n.sub.O is selected in such a way that it more or less coincides with the refractive index n.sub.M (.tbd.n.sub.m) of the polymeric matrix. In the case of network systems, an adjustment of the refractive indices owing to the customarily very much higher liquid crystal content in the light-modulating layer is not absolutely necessary, but can be carried out to increase the light transmission and the contrast. An electrically switchable light scattering effect is observed in these electrooptical liquid crystal systems.
If no voltage is applied to the electrodes, between which the matrix or the network is customarily arranged like a sandwich, light incident on the statistically aligned liquid crystal molecules is strongly scattered and the system is non-transparent. On applying a voltage, the liquid crystal molecules are aligned parallel to the field and perpendicular to the E vector of the transmission light.
In the case of microdroplets matrix systems, perpendicularly incident light sees an optically isotropic medium when voltage is applied owing to the adjustment of n.sub.O and n.sub.M and the system appears transparent. An adjustment is necessary in order to avoid a scattering of the light at the matrix/liquid crystal droplets phase boundary. EP 0,272,585 describes another embodiment in which the refractive index n.sub.x, which the liquid crystal exhibits at completely statistical orientation, is adjusted to the refractive index of the matrix n.sub.M. In this case, the system is transparent in the field-free state, and it is converted into the opaque state by applying a voltage.
In the case of network systems, an adjustment of the refractive indices is not absolutely necessary, as owing to the high liquid crystal content in the material of the light-modulating layer, the scattering at the network/liquid crystal phase boundary is obviously less strong. In the switched-on state, the system appears transparent even without adjustment of the refractive indices. In the case of network systems, the use of liquid crystals having high optical anisotropy is preferred to achieve a transmission which is as low as possible in the off-state.
In WO 89/09807, the use of an optically anisotropic, for example, liquid crystalline polymeric matrix material has been proposed in order to avoid the frequently observed clouding ("haze", in particular "off-axis haze") in the transparent state of the system. In systems of this type, the refractive indices of liquid crystal and optically anisotropic matrix can be adjusted to each other so that the transparent state is obtained either with the voltage applied or switched off.
Electrooptical liquid crystal systems according to the above description have been especially proposed for large-surface-area indicating systems, for architectural applications (windows, room dividers, sunroofs etc.) and for motor vehicles (windows, sunroofs etc.), these systems also being suitable for temperature regulation by virtue of controlled screening of the solar radiation. They can be switched on by applying a direct or alternating voltage.
As these systems are, in particular, also intended for "out-door" applications, liquid crystal mixtures are required which are characterized by a high clear point, high .DELTA..epsilon., a broad nematic range, a favorable temperature dependence of the electrooptical parameters and a high stability to UV and temperature.
Examples of other applications are:
A serious disadvantage of the conventional electrooptical systems described above is that the liquid crystal mixture or individual components of the liquid crystal mixture are, in many cases, distinguished by too high a solubility and/or too high a temperature dependence of solubility in the cured matrix-forming polymer.
If, for example, the solubility and/or the temperature-dependence of the solubility of one or several components differs quite significantly from that of the remaining components, it may happen that the physical properties of the mixture and in particular also of the refractive indices n.sub.e and n.sub.o are substantially affected, which disturbs the adjustment of n.sub.o or n.sub.e or another refractive index of the liquid crystal mixture to n.sub.M, thus resulting in a considerable deterioration of the optical properties of the system. The incorporation of part of the liquid crystal mixture into the matrix furthermore leads to a reduction of contrast.
A high solubility of the liquid crystal mixture is especially disadvantageous in dyed electrooptical systems. The pleochroitic dye which is dissolved in the liquid crystal mixture, is incorporated into the cured polymer matrix which therefore exhibits a permanent and not an electrically switchable coloration.
The liquid crystal mixtures used hitherto only inadequately fulfill the requirements outlined above and, in particular, there is a great demand for electrooptical systems which are characterized by a low solubility of the liquid crystal mixture in the cured polymer matrix.